Nanometer size or so-called Quantum-dot size particle composites can exhibit large optical nonlinearities which make them of interest for many applications such as optically addressed switching and thresholding. They are also inexpensive and could support array addressing. An important advantage of these kinds of nonlinear media is that they can be produced in bulk and one has control over the particle type, size, shape and concentration. One can also select the host medium in which the particles are placed. Further, depending on the particular physical mechanism exploited for the bulk material's nonlinear properties, one has a choice of response times from picoseconds to seconds. Indeed, by engineering a material to have nonlinear properties arising from two different physical mechanisms, bistable behavior is expected. These many degrees of freedom permit the fine tuning of the optical nonlinearity in order to tailor it for specific applications. This produces a degree of flexibility not offered by alternative nonlinear media.
Biomolecular self-assembly holds the potential for developing advanced material properties in semiconductors and metals by controlling crystal structure, crystal size, and orientation of crystal growth. This control is important because many electrical and optical properties are highly anisotropic or size dependent. Biomineralization is one aspect of biomolecular self-assembly where there is a consensus among workers that general principles apply to most systems through the use of a polymer matrix and charged nucleation sites to control crystal growth. Shells, teeth, and bone are constructed in this manner. In mammalian teeth, calcium phosphate is organized as long thin needles in an acidic protein matrix. Biological control of inorganic nanoparticle formation has recently been shown to occur as a byproduct of cadmium detoxification in the yeasts Candida glabrata and Schizosaccharomyces pombe. An in vitro, process of producing nanoparticle CdS was demonstrated in 1990 utilizing short alpha linkage peptides (.alpha.-glutamic acid-cysteine).sub.4 -gly. These nanoparticles have been characterized and show a sharp blue-shifted absorption peak at 295 nm indicative of quantum confinement and particle uniformity. A major limitation of this biomolecular approach is the inability to tailor the particles for specific sizes and economic feasibility. Nanometer size particles have been found in polymers, glass and zeolites.
Two different methods have been developed to form nanometer-size particles within polymers. In the first method a soluble metal salt is co-dissolved in a suitable solvent along with a polymer which is then cast into a film. In the second method metal ions are ion-exchanged into a solid polymer. Precipitation within the polymer (H.sub.2 S, NaOH) generates nanoparticles. Cadmium and lead sulfide have been formed in Nafion using the ion-exchange technique. This basic process has been extended recently to the formation of .gamma.-Fe.sub.2 O.sub.3 nanoparticles that resulted in transparent magnetic materials. Iron salts were ion-exchanged into commercial ion-exchange resins and subsequently precipitated by intermediate hydroxide formation and further oxidation. Each method results in a polydisperse nanoparticle distribution.
Borosilicate glass containing nanoparticles of CdS.sub.x Se.sub.1-x has been available commercially from Schott and Corning for many years. A rigid matrix inhibits particle coalescence that results in a polydisperse nanoparticle distribution.
The three-dimensional porous crystalline host, zeolites, have been utilized for nanoparticle formation. Particle size can be tailored by a change in zeolite chemistry which results in different size pore formation. Disadvantages include poor control of the fabrication process, instability in many solvents, and small crystal size which is unsuitable for Non-Linear Optical applications.
Much interest was aroused in nanoparticles several years ago, because it was realized that such particles no longer exhibit the optical properties of the bulk material, but have size dependent properties. One can assume that the electrons or excitons in the material have momenta confined in the same way as for an elementary quantum mechanical particle-in-a-box problem. Quantum confinement is thus expected when a particle's radius is smaller than the size of the Bohr radius of the electron in the material from which the particle is made. Bohr radii can range from 0.1 Angstroms in a hydrogen atom to 80 Angstroms in germanium.
Interest in the use of microparticles for optical nonlinear media originated from the work of Jain and Lind in 1983 (J. Opt. Soc. Amer., 73:647 (1983)). They found this nonlinearity present in commercially available sharp-cut-off filters (e.g. from Corning or Schott) which contained nanocrystals of CdS.sub.x Se.sub.1-x. These composite glasses have been found to have a third order nonlinearity, .chi..sup.3, of the order of 10.sup.-8 esu to 10.sup.-9 esu and response times of 1 ps to 0.1 ps. Several groups have reported larger nonlinearities from semiconductor copper chloride in glass with particle sizes ranging from 2.5 nm to 10 nm. Applications for such materials are to ultrahigh speed light controlled switching devices. Also, using metallic particles coated with or embedded in optically nonlinear dielectrics, each part of the structure has intrinsic nonlinearities and the one can be made to enhance the other, giving a predicted .chi..sup.3 as large as 10.sup.-2 esu.
The first theoretical investigations of quantum confinement in semiconductor microcrystallites were reported by Efros and Efros and by Brus in the 1980's. They promoted the idea that by controlling the band-gap in materials, one can tune the wavelength of light emitted or absorbed. The band-gap depends upon particle size and shape and the overall optical properties depend upon particle number density, particle material properties and host fluid properties. A detailed theoretical analysis to determine the linear and third order susceptibilities of a small metal sphere was derived in 1986 under the assumption that a quantum confinement effect would dominate the optical properties. A spherical box model was adopted for the potential and the one-electron Schrodinger equation was solved at absolute zero. The susceptibilities were calculated using the density matrix theory of Butcher and MacLean (Proc. Phys. Soc., 81:213 (1963)). Their final expression for .chi..sup.3 takes the form EQU .chi..sup.3 =.mu.(1/r.sup.3)(e.sup.4 /[m.sup.2 h.sup.5 w.sup.7 ]) (1-r/r.sub.0)
Note that this particular mechanism suggests that .chi..sup.3 increases as the cube of the inverse radius of the particle.
For this mechanism, the origin of the third order nonlinear susceptibility arises from the material's behavior as simple two level absorber. The absorption saturates when the upper state is half occupied and the refractive index becomes intensity dependent. One can associate an absorption profile for the material with a refractive index modulation, as determined by the Kramers-Kronig dispersion relations. With a limited number of participants in this absorption process, the absorption can be saturated and an intensity dependent refractive index variation will result. It is expected that this mechanism can become very large if the particle size distribution is small, leading to a very narrow absorption bandwidth and associated very large change in refractive index. Hence the wide interest in a method by which a monodisperse nanoparticle distribution can be fabricated.
As mentioned above, local field effects can provide a degree of feedback to the particle's behavior. By changing the refractive index of the particle, a further enhancement or reduction of the field within the particle occurs due to dielectric confinement variations. For small detunings from resonance, the field may be concentrated or expelled from the particle, depending on the change in refractive index. Hence the intensity inside the particle is modified which in turn changes the refractive index due to the nonlinearity. It is this feedback that can lead to regions of optical bistability. i.e. to intrinsic bistability.
These are effects which have not been explicitly observed, but are expected; once again, it is the lack of the very uniform size distribution of particles with a narrow absorption peak that is thought to be the experimental limitation. Consequently, a long standing need exists for a nanoparticle size particle composite with uniform particle size distribution and a narrow absorption peak.